But he's not waging a battle with grenades or guns. Instead, the 40-year-old Passaic, N.J., resident is waging a kindness crusade. An idealist by nature, Greenbaum believes that ordinary citizens can destroy terrorism through extraordinary acts of goodness.

His year-old organization, Partners in Kindness, encourages kindness through a plethora of e-mailings, lectures, posters and contests. More than 5,000 members on six continents subscribe to Greenbaum's weekly "Kind Words" e-mails. His readers - some from as far away as Iran, Kuwait and Japan - report their daily acts of kindness via e-mail to Greenbaum's Web site.

Schools, organizations and radio programs worldwide have taken up the cause.

It also has caught on at New York City Transit, where Greenbaum works as a computer specialist. Thanks to his efforts, New York City Transit recently started a kindness public awareness campaign among employees. Greenbaum now is trying to push a "Heroes of Kindness" poster program in which posters depicting New York City Transit employees and passengers performing inspiring acts of kindness would be plastered throughout the system.

"What better place is there to have the undivided attention of millions of New Yorkers?" Greenbaum asked. "It's a great opportunity to inspire them with stories that motivate them to do acts of kindness."

Paul Fleuranges, vice president of public affairs at New York City Transit, said Greenbaum has made a dramatic impact on the organization. "We've featured him in our newsletter and have posters up throughout the building promoting his Web site and message of kindness. We are encouraging others here to follow in his footsteps," Fleuranges said.

Greenbaum, who works on the voluntary project in his spare time, said it keeps him going.

"It's therapeutic," he said. "I come home from work and I go straight to the computer. My readers give me tremendous feedback, saying how it inspires them."

One user, Lin Marelic of Mission Viejo, Calif., said that the Kindness e-mails are "an important part of my life. With all the horror in the world, they inspire me with a positive message."

Fayge Young, a homemaker from Oak Park, Mich., called the e-mailings "my daily booster shot to keep me upbeat. If someone in his (Greenbaum's) situation can have the energy to do something like this, I can draw on the same kind of power."

Though Greenbaum cannot attest to the veracity of every story submitted, he ensures that the person writing and sending the e-mail saw the act of kindness firsthand.

He quotes a New York Times article about medical research that found that acts of kindness stimulate the brain in the same place that physical pleasures do.

"Medical research has shown that doing kindness causes enjoyment," Greenbaum said joyfully. "This is just one more benefit."

If the research is to be believed, then Greenbaum should be a very happy man. He has regularly performed acts of kindness, having served as a "Big Brother" for children from broken homes, provided computer training for the unemployed in his community, and visited nursing homes on a regular basis.

At his Manhattan office each day, he makes an effort to greet everyone he encounters jubilantly. When they inquire why he's so happy, he replies, "Because it makes you live longer."

"This usually provokes them to smile, too," he said. "After a while, they start greeting me first, even though they don't even know my name. It's terrific. That means it's working."

Greenbaum insists that his is not a courtesy campaign.

"Courtesy is very different from kindness," Greenbaum said. "You can be very courteous but hateful. My objective is to teach people to care about each other."

He avoids listening to news about suicide bombings in Israel because he finds it too upsetting. But even in the throes of violence, he sees a mission for himself.

"Many of the people on my list live in Israel, and they tell me all the time that my e-mails are very comforting," he said.

Much like music or art, kindness requires practice, he points out. Greenbaum wants people to become experts. His crusade is an apt tribute to Shoshana, his wife of 15 months.

"Kindness was what she was all about," Greenbaum said.

At the Long Beach, N.Y., elementary school where she taught, "She always greeted her students with a big smile that made them feel how much she cared for them," he recalled. "She tried to connect with every student."

She even gave her students her phone number, urging them to call her whenever they needed advice or help. And to everyone she encountered, he said, "She filled the world with a smile that radiated her love."

What impressed him most about his wife, Greenbaum recalled, was her keen intelligence and desire to do good.

"That's what we had in common was our desire to help people and bring love into the world," he said. "And that's what these programs are doing."

The 31-year-old teacher was five months pregnant when she was invited to an all-expenses paid summer program in Israel as part of her master's degree program in education. Shoshana was elated, Greenbaum recalled. "She loved Israel. She wasn't afraid at all."

Greenbaum stayed with her for several weeks in Israel and then returned home to his job. On Aug. 9, four days after Greenbaum's departure, Shoshana went to Sbarro for lunch and never came out.

A terrorist detonated a bomb in the restaurant, killing 15 people, including Shoshana, an only child.

Greenbaum, who married late in life because he wanted to make sure he found the right person, felt his life had been shattered. He cried. He grieved. He questioned G-d. But he refused to get angry.

"I knew that G-d was sending me a message, but I didn't know what it was," he said.

Greenbaum prayed that G-d would help him make the world better, and to prevent such tragedies from happening again.

Today he feels his prayers were answered.

Although he thinks of Shoshana every day, he is able to live through his pain because of his quest.

"From the response I'm getting, I see that it's having a profound effect on people all over the world of all religions. I am fighting hatred with kindness, one person at a time," he said. "That's giving me tremendous strength."